<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f(1)">
<input type="button" value="减" onclick="f(2)">
<input type="button" value="乘" onclick="f(3)">
<input type="button" value="除" onclick="f(4)">
<div>
    结果:<span></span>
</div>
<script>
    function f(x) {
        //得到页面中相关的元素对象
        let i1 = document.querySelector("#i1");
        let i2 = document.querySelector("#i2");
        let s = document.querySelector("span");
        //判断输入的两个值是不是NaN
        if (isNaN(i1.value)||isNaN(i2.value)){
            s.innerText="输入错误!";
            return;
        }
        switch (x) {
            case 1: //加
                s.innerText = i1.value*1+i2.value*1;
                break;
            case 2: //减
                s.innerText = i1.value-i2.value;
                break;
            case 3: //乘
                s.innerText = i1.value*i2.value;
                break;
            case 4: //除
                s.innerText = i1.value/i2.value;
                break;
        }
    }
</script>
</body>
</html>